Mechanical fasteners, such as bolts, nuts and the like once set, may loosen due to vibrations or thermal expansion or contractions. This phenomenon is often referred to as “self loosening”.
Several approaches of preventing self loosening, thereby locking fasteners set to an axial preload are known. For example, lock washers may be used to increase the frictional forces between a fastener and its underlying substrate. However, with time, and external forces fasteners secured by lock washers may also loosen, and may undesirably damage or scour the underlying substrate. As such, lockwashers have fallen out of favour in the fastener industry.
Another approach is to apply a chemical adhesive. However, lubricants can typically not be used with such chemical adhesives, making precise pre-loading difficult, if not impossible. Further, such chemical adhesives are sensitive to contaminants; have a limited shelf-life; and need to be re-applied periodically. While high strength chemical adhesives may be effective when properly selected and applied, they must typically be heated to very high temperatures to allow removal/loosening of fasteners. While fasteners installed with low to mild strength to mid-strength adhesives may be disassembled by hand, they may allow self-loosening with time.
Other approaches rely on using a deformed fastener that locks as a result of its deformation. As a result, torque applied to the fastener creates an axial load on the fastener and deforms the locking mechanism, and the axial preload load on the fastener is somewhat less than it would be in the absence of the locking mechanism. How much less will depend on the force required to deform the locking mechanism, and this amount may not readily be known or be repeatable with accuracy. Further, deformation may waken the fastener.
Still yet another approach uses a castellated nut, and cotter pin, with a complementary hole drilled through the threaded fastener after the nut has been set to the desired load. This, however, requires the fastener to be accessible, typically to a drill, and may be labour intensive requiring substantial skill. Further, clearance around the pin may allow play between the fastener and pin resulting in pin failure.
Another approach uses a lockwire passed through transverse holes in at least two threaded fasteners and twisted back on itself in a specific pattern. Again, lockwiring is labour intensive, time consuming and requires significant expertise in proper routing of the wires. Often the quality of lockwiring is unsatisfactory due to unintended variations in the taughtness of the twists and the tension on the lockwires. When wired incorrectly, a lockwire may actually advance or even initiate self loosening of the fasteners.
Still other approaches rely on specific fasteners that have locking heads that lock in a particular orientation. Such fasteners may be set to an initial specific pre-load, and then further tightened or loosened to move the head into its locking orientation. However, the resulting load used to achieve the locking orientation may not correspond to the desired pre-load.
Thus, there remains a need for a positive-locking fastener that achieve a desired axial preload.